1. Field of the Invention
The present invention relates to a method and device for sensing a magnetic field with use of a magneto-resistive property of a supercoductive material. The superconductive material contains grain boundaries.
2. Description of the Prior Art
Conventionally, a magnetic sensor which utilizes the Hall effect of magneto-resistive effect in a semiconductor or a magnetic sensor which utilizes the magneto-resistive effect in a magnetic material is widely used for sensing or measuring a magnetic field. These sensors have a limit of resolution of sensing a magnetic field of about 10.sup.-3 gauss. Conventionally, for sensing a weak magnetic field, SQUID (superconductive Quantum Interference Device) has been used. However, SQUID needs the Josephson junction which utilizes thin insulation layer between two thin superconductors. SQUID devices require a high production technology for example because of the thin insulation layer is necessary and it is not easy to produce the SQUID devices. In addition, in order to sense a magnetic field, it is necessary to cool the SQUID, to eliminate noise and to process the output signals from the SQUID, therefore to handle the SQUID for the measurement of the magnetic field is not easy.
The inventors of the present invention have found that the superconductive state of a ceramic superconductive element comes to break the superconductive state by application of a small magnetic field due to the grain boundary property of the superconductive material. Also there is a resistance which increases abruptly corresponding to increment of the applied magnetic field and they propose a new magnetic field sensor utilizing the magneto-resistive effect in the U.S. patent application, Ser. No. 226,067, now U.S. Pat. No. 5,011,818 issued Apr. 30, 1991. The magnetic sensor using the ceramic superconductor mentioned above is expected, as the superconductor magnetic sensor having a high sensitivity with easy operation and handling. In addition this sensor is easy to produce partly because the sensor has a large resistivity increasing factor at the time of application of such a magnetic field. Accordingly the superconductive condition is broken and the structure of the device is simple and the control of the sensitivity of the device can be easily made by the application of current, partly because the measurement output can be easily obtained.
However, the property of the superconductor magneto-resistive element as mentioned above is non linear in terms of the resistance and the applied magnetic field. Accordingly, it is difficult to calculate an absolute intensity of an applied magnetic field based on the resistance value of the element.
Besides, there have been used various kinds of magnetic field sensor using high speed electron mobility semiconductors such as In Sb and In As, and ferro magnetic metal such as Fe-Ni, Co-Ni.
However, with the conventional magnetic field sensor using the above material it is difficult to sense a weak magnetic field accurately.